Recently, the necessary light source of a liquid crystal display (LCD) used in personal computers, notebook computers, personal digital assistants (PDA's) and web-pads utilizes a driving device to drive a load (e.g. a cold cathode fluorescent lamp) of a high voltage such that the light is emitted to a backlight plate and a user can clearly view a displayed image.
Particularly, an LCD TV or the LCD with a touch screen needs higher brightness to compensate for visual necessity. As the load is driven to a high voltage, the brightness is higher when the lamp current of the load is greater. In order to enhance brightness and uniformity, it is necessary to use a plurality of loads and simultaneously pay much attention to whether the lamp current is uniform or whether the error between the properties of the loads is small. Furthermore, the additional loads lead to increasing the number of driving units that drive the loads and increase the volume of a circuit board and difficultly of manufacture. Simultaneously, the manufacturing cost is increased. When the loads are utilized to compensate the brightness and uniformity, the lamp current and the brightness often are not uniform because of the error between the loads. Thus, it is difficult to select the loads or it is necessary to have more loads to improve the brightness and uniformity increasing the difficultly of manufacture and adjustment. Simultaneously, the manufacturing cost is increased. The lamp current control method according to the load is described as follows:
(1) A typical driving device t emits light and is shown in FIG. 1 includes a power unit 10, a control unit 11, a driving unit 12, a voltage step-up unit 13 and a load 14. When the power unit 10 is started up to input a voltage, the driving unit instantly drives the voltage step-up unit 13 by means of a converse and direct piezoelectric effect and then the voltage step-up unit 13 drives the load 14 to emit light. The control unit 11 outputs a resonant frequency to control the average current of the load 14 by sensing the feedback lamp current of the load 14, and therefore the generated light is emits behind the image display of a backlight plate. The voltage step-up unit 13 utilizes a push-pull type structure to drive the load 14, and the voltage step-up unit 13 is a ceramics transformer. However, the load 14 in the prior art often has problems, such as unstable electrode, gas, etc., such that the lamp current in the load 14 has abnormal behaviors (e.g. power loss). Although a feedback loop is connected to the control unit 11, the control unit 11 still cannot control the lamp current of the load 14 because the load 14 must be driven to emit the light in the high voltage. When the load 14 is driven to emit the light in the high voltage, the load 14 generates the lamp current with high energy to generate high frequency noises or arc light effect, etc. For above reason, it leads to the power loss inside the load 14 or the damage of the load 14.
(2) Referring FIG. 2, according to the problem show in FIG. 1, another prior art utilizes current control mode to sense the flow rate of the lamp current of the load 14. A current sensing unit 15 that consists of current generators in series connection sensing current signals of the load 14, then processing comparisons between two voltage drops through a signal processing unit 16 to generate a compensating signal which is fed back to the control unit 11. There must be a plurality of short lamp in series connection applied in the prior art shown in FIG. 2, but a single long lamp is not suitably applied in the prior art shown in FIG. 2. In addition, the current sensing unit 15 must be disposed around the load 14 in the prior art shown in FIG. 2, such that the end of the load 14 has low brightness that becomes lower than before. For the above reason, a dark point is displayed, and is generated by the location of the current sensing unit 15.
Accordingly, there exists a need for a lamp current control circuit to solve the above-mentioned problems and disadvantages.